Method and apparatus for blanking T-waves from combipolar atrial cardiac signals based on expected T-wave locations

ABSTRACT

The stimulation device blanks T-waves from the atrial channel of an electrical cardiac signal by employing a T-wave blanking interval localized to the expected location and duration of the T-wave. To this end, the stimulation device determines the average interval between an R-wave and a T-wave in the patient in which the device is implanted and also determines the average duration of a T-wave within the patient. A T-wave blanking interval is initiated following the average R-T interval subsequent to detection of an R-wave and lasts for a period of time equal to the average T-wave duration. In this manner, highly localized T-wave blanking is achieved permitting P-waves or other atrial signals to be detected during remaining non-blanked portions of the atrial channel of the cardiac signal at least for the purposes of atrial rate detection. The relatively short T-wave blanking interval of the invention is particularly well suited for use in combipolar sensing systems. Method and apparatus implementations are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to commonly-assigned U.S. patent applicationSer. No. 09/354,244, filed Oct. 25, 2001, titled METHOD AND APPARATUSFOR BLANKING T-WAVES FROM COMBIPOLAR ATRIAL CARDIAC SIGNALS BASED ONEXPECTED T-WAVE LOCATIONS, now issued as U.S. Pat. No. 6,539,259.

FIELD OF THE INVENTION

The invention generally relates to implantable cardiac stimulationdevices, such as pacemakers or implantable cardioverter-defibrillators(“ICDs”) and, in particular, to techniques for processing electricalcardiac signals detected using combipolar sensing.

BACKGROUND OF THE INVENTION

A pacemaker is a medical device, typically implanted within a patient,which recognizes various dysrhythmias such as an abnormally slow heartrate (bradycardia) or an abnormally fast heart rate (tachycardia) anddelivers electrical pacing pulses to the heart in an effort to remedythe dysrhythmias. An ICD is a device, also implantable into a patient,which additionally recognizes atrial fibrillation (AF) or ventricularfibrillation (VF) and delivers electrical shocks to terminatefibrillation.

Pacemakers and ICD's carefully monitor characteristics of the heart suchas the heart rate to detect dysrhythmias, discriminate among differenttypes of dysrhythmias, identify appropriate therapy, and determine whento administer the therapy. The heart rate is tracked by the device byexamining electrical signals that are manifest concurrent with thecontraction and expansion of the chambers of the heart. The contractionof atrial muscle tissue is manifest by the generation of a P-wave. Thecontraction of ventricular muscle tissue is manifest by the generationof an R-wave (sometimes referred to as the “QRS complex”). Expansion ofthe ventricular tissue is manifest as a T-wave. Expansion of the atrialtissue usually does not result in a detectable signal. The sequence ofelectrical events that represent P-waves, followed by R-waves (or QRScomplexes), followed by T-waves are sensed using sensing leads implantedinside the heart, e.g., sensing leads.

One commonly used type of sensing lead is the unipolar lead, whichincludes a single electrode at its tip. The device detects electricalvoltage differentials between the electrode and the external body of thedevice itself. Typically, one unipolar lead is inserted within the atriaand another within the ventricles, from which the device derivesseparate atrial and ventricular channel cardiac signals. Anothercommonly employed type of sensing lead is the bipolar lead wherein thelead includes two electrodes mounted near its tip. The device detectselectrical voltage differentials between the two electrodes. Again,typically, one lead is inserted within the atria and another within theventricles, from which the device derives separate atrial andventricular channels of cardiac signals.

A common problem with unipolar leads is that, because the device issensing voltage differentials between the tip of the lead and the bodyof the device, significant far-field electrical signals are detectedalong with the intended atrial or ventricular cardiac signals. A“far-field” signal is a signal originating far from the sensor of thesensing lead, but detected by the sensing lead nonetheless. For example,the atrial cardiac signal derived from the atrial lead will typicallyinclude significant ventricular signals. A significant advantage of thebipolar lead is that, because electrical voltage differentials aredetected only between two electrodes located closely adjacent to oneanother at the end of the lead, far-field sensing is significantlyreduced. However, bipolar leads are more expensive and are generallyperceived as being less reliable than unipolar leads and hence are notpreferred by all physicians.

In an attempt to provide the advantages of bipolar sensing usingunipolar leads, some state-of-the-art devices employ combipolar sensingtechniques. With combipolar sensing, a pair of unipolar leads aremounted within the heart, one in the atria and one in the ventricles. Aventricular channel cardiac signal is generated in the same manner aswith conventional unipolar sensing wherein electrical voltagedifferentials are detected between the tip of the ventricular lead andthe body of the device. However, the atrial channel of the cardiacsignal is generated by detecting voltage differentials between theelectrodes at the tips of the atrial and ventricular leads. For a morecomplete description of combipolar systems, see U.S. Pat. No. 5,522,855(Hognelid), incorporated herein by reference.

With combipolar sensing, because the atrial channel is derived basedupon voltage differentials between the tips of the two unipolar leads,improved detection of atrial signals is achieved as compared withsystems which require the relatively weak atrial electrical signals tobe detected based upon voltage differentials generated between the tipof the atrial lead and the body of the device. Ventricular electricalsignals are typically much greater in magnitude than atrial signals,hence, with the combipolar sensing technique, it is sufficient to sensethe ventricular signals based upon voltage differentials generatedbetween the tip of the ventricular lead and the body of the device.Hence, an overall improvement in the sensitivity of the detection ofatrial signals is achieved using combipolar sensing, yet the perceivedbenefits of unipolar leads are retained, namely that the leads are lessexpensive and more reliable.

Thus, combipolar sensing provides many advantages. One disadvantage,however, is that, because the atrial channel is detected based uponvoltage differentials between the tips of the atrial and ventricularleads, ventricular signals are sensed as “near-field” signals. As aresult, ventricular signals may have a greater magnitude on the atrialchannel than the atrial signals. Hence it may be difficult to filter theventricular signals from the atrial channel. (The ventricular channel,because it is detected based upon voltage differentials between the tipof the ventricular lead and the body of the device, may also pick upfar-field atrial signals, but these are typically very weak as comparedto the ventricular signals and hence can easily be filtered out.)

Regardless of the electrode configuration being used, there is a needfor the implanted device to be able to readily and reliably distinguishbetween various electrical events such as P-waves, R-waves and T-waves.For example, it is of critical importance that the device be capable ofrecognizing the occurrence of certain atrial arrhythmias based on thesensed atrial rate, and in determining such rate it is criticallyimportant that neither R-waves nor T-waves be falsely sensed as aP-wave. Such may be particularly problematic when an combipolarelectrode configuration is being used because, as noted, P-waves,R-waves, and T-waves may be sensed as being of the same order ofmagnitude on the atrial channel. This problem exacerbated during anautomatic mode switch (AMS), e.g., when switching the device from a DDDmode to a VVI or DDI mode. DDD, VVI and DDI are standard device codeswhich identify the mode of operation of the device. DDD indicates adevice which senses and paces in both the atria and the ventricles andis capable of both triggering and inhibiting functions based upon sensedevents. VVI indicates that the device is capable of pacing and sensingonly within the ventricle and is only capable of inhibiting thefunctions based upon sensed events. DDI is identical to DDD except thatthe device is only capable of inhibiting functions based upon sensedevents, rather than triggering functions. Numerous other device modes ofoperation are possible, each represented by standard abbreviations ofthis type.

One technique commonly employed for processing the atrial or ventricularchannel signals to eliminate unwanted signals uses “blanking intervals”.With a blanking interval, the device does not process electrical signalsduring a predetermined interval of time either for all device functions(absolute blanking) or for selected device functions (relativeblanking). As one example of absolute blanking, upon detection of anR-wave on the ventricular channel, the device will not detect anysignals on the atrial channel during a post ventricular atrial blanking(PVAB) interval. The atrial blanking interval is provided to prevent thedevice from erroneously responding to a far-field R-wave on the atrialchannel. As one example of relative blanking, upon detection of anR-wave on the ventricular channel, the device will ignore all signalsdetected on the atrial channel during a post-ventricular atrialrefractory period (PVARP) as far as the triggering or inhibiting ofpacing functions is concerned, but not for other functions such asdetecting and recording diagnostic information, particularly detectionof premature atrial contractions (PACs). Pacemakers and ICDs may employboth the PVAB and the PVARP, with the PVAB being much shorter than thePVARP interval.

The effect PVAB and PVARP intervals is illustrated in FIG. 1 which showsa stylized representation of one atrial and one ventricular cardiacchannel of a normal sinus rhythm detected using combipolar sensing.(Actual devices typically employ multiple atrial and ventricularchannels to track different types of information. For example, oneatrial channel may be employed for bradycardia detection, whereasanother is employed for controlling AMS operations. For clarity indescribing the effect of the PVAB and PVARP intervals, FIG. 1illustrates only a single atrial channel and a single ventricularchannel). The ventricular channel includes R-waves and T-waves. Theatrial channel includes P-waves as well as ventricular T-waves, detectedas near-field waves. FIG. 1 also illustrates the PVAB and PVARPintervals applied to the atrial channel. The PVAB interval, which beginsupon detection of an R-wave on the ventricular channel, is set to aduration sufficient to cover the R-wave such that the R-wave is notdetected in the atrial channel. The PVARP blanking interval is set to alength such that the T-wave, although detected on the atrial channel, isignored. Hence, within the atrial channel, for the purposes of atrialrate detection, only events detected outside of the PVARP interval areused for the rate calculation. With proper setting of the PVARP and PVABintervals, only P-waves are typically detected, and hence an accuratecalculation of the true atrial rate is achieved for normal sinus rhythm.

Hence, blanking schemes may be used to blank T-waves from the atrialchannel to prevent such T-waves from being falsely sensed as P-waves.However, such blanking schemes have proven less than satisfactorybecause P-waves may occur during the blanking intervals. Hence, thedevice may significantly underestimate the true atrial rate, and therebyfail to detect the tachyarrhythmia, flutter or fibrillation occurring inthe atria. Thus, improper therapy may be administered.

FIG. 2 illustrates stylized atrial and ventricular channel cardiacsignals detected using combipolar sensing during an episode of atrialtachyarrhythmia. As can be seen from the atrial cardiac signal, thefrequency of P-waves is far greater than in FIG. 1. As a result, atleast two P-waves occur during the PVARP interval, which are thereforeignored for the purposes of atrial rate detection. As a result, theatrial rate is substantially underestimated, possibly resulting infailure of the implantable device to administer appropriateanti-tachycardia pacing. FIG. 2 illustrates an extreme situation whereina significant underestimation of the atrial rate can occur. But even inmore benign circumstances, occasional P-waves may be blanked by thePVARP interval resulting in a slight underestimation of the atrial rate,perhaps sufficient to trigger an unnecessary mode switching or the like.

One solution that has been proposed for providing a better estimate ofthe atrial rate is to include events detected during the PVARP for thepurposes of atrial rate calculations. Using this technique, P-wavesdetected during the PVARP are thereby sensed. However, T-waves may alsobe sensed during the PVARP interval resulting in an overestimation ofthe atrial rate, perhaps sufficient to trigger unnecessaryanti-tachyarrhythmia therapy or at least sufficient to trigger anunnecessary mode switch.

Hence, there is a significant need to provide an improved technique fordetermining the true atrial rate. This need is particularly significantwhen using combipolar sensing, but also arises when using otherelectrode configurations such as conventional unipolar or bipolarsensing where T-waves may be detected on an atrial channel as far-fieldevents.

SUMMARY OF THE INVENTION

In accordance with the invention, a method is provided for generating aT-wave blanking interval using an implantable cardiac stimulationdevice. In accordance with the method, the stimulation device senses acardiac signal, identifies expected locations and durations of T-waveswithin the cardiac signal, and then blanks a portion of an atrialchannel of the cardiac signal to ignore signals occurring within aperiod of time corresponding to the expected locations and durations ofthe T-waves. By localizing the T-wave in this manner, then blanking itfrom the atrial channel from which the atrial rate is derived, a morecorrect determination of the true atrial rate can be obtained.

Within an exemplary embodiment, the cardiac signal processed by thestimulation device includes electrical signals output from senseamplifiers of the stimulation device or includes intracardiacelectrogram (IEGM) signals. The step of identifying the expectedlocations and durations of T-waves within the cardiac signal includesthe steps of identifying an R-wave in the cardiac signal, tracking afirst predetermined interval of time subsequent to the R-wave, which isrepresentative of the expected interval between an R-wave and thebeginning of a successive T-wave, and tracking a second predeterminedinterval of time subsequent to the beginning of the T-wave, which isrepresentative of the expected duration of the T-wave. The expectedinterval between the R-wave and the T-wave (i.e., the R-T interval) andthe expected duration of the T-wave are determined in advance bymeasuring and averaging R-T intervals and T-wave durations for a set ofR-wave/T-wave pairs. Preferably, statistical information representativeof a large number of R-wave/T-wave pairs is maintained, includinginformation pertaining to the minimum and maximum voltages of theR-waves and T-waves, the durations of the R-waves and T-waves, and theintervals between the R-waves and T-waves. Also, preferably, thestatistical information includes averages and standard deviations forthese values. Periodically, to accommodate possible changes in the R-Tinterval and T-wave duration, additional R-wave/T-wave pairs areanalyzed and the statistical information is adjusted accordingly. Also,preferably, before actually modifying the statistical information, adetermination is made as to whether the characteristics of the newlydetected R-wave/T-wave pairs deviate from the recorded average by nomore than a predetermined amount, such as by no more than the firststandard deviation. In this manner, the statistical information, fromwhich the expected locations and durations of the T-waves aredetermined, is not improperly adjusted based upon anomalousR-wave/T-wave pairs, perhaps detected during flutter or fibrillation.

Apparatus embodiments of the invention are also provided. Other objects,features and advantages of the invention will be apparent from thedetailed descriptions, which follow in combination with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention may be morereadily understood by reference to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a graph illustrating exemplary atrial and ventricular channelsof cardiac signals detected using combipolar sensing during normal sinusrhythm.

FIG. 2 is a graph illustrating exemplary atrial and ventricular channelsof cardiac signals detected using combipolar sensing during an episodeof atrial tachyarrhythmia.

FIG. 3 is a functional block diagram of a dual-chamber implantablestimulation device illustrating the basic elements of a stimulationdevice which can provide cardioversion, defibrillation and pacingstimulation;

FIG. 4 is a flow chart illustrating a method performed by the system ofFIG. 3 for blanking T-waves from an atrial channel cardiac signal basedupon predetermined information identifying the location and duration ofthe T-wave relative to a preceding R-wave.

FIG. 5 is a graph illustrating exemplary atrial and ventricular channelsof cardiac signals detected during a period of atrial tachyarrhythmiausing combipolar sensing and specifically illustrating a T-wave blankinginterval set to the expected location of T-waves within the cardiacsignals.

FIG. 6 is a flow chart illustrating a method performed by the system ofFIG. 3 for determining the expected location and duration of T-wavesrelative to R-waves for use by the method of FIG. 4.

FIG. 7 is a flow chart illustrating a state machine employed by acontroller of the system of FIG. 3 for detecting R-waves, then blankingT-waves in accordance with the general method illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forpracticing the invention. This description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

In FIG. 3, a simplified block diagram is shown of a dual-chamberimplantable stimulation device 10 which is capable of treating both fastand slow arrhythmias with stimulation therapy, including cardioversion,defibrillation, and pacing stimulation. While a dual-chamber device isshown, this is for illustration purposes only, and one of skill in theart could readily eliminate or disable the appropriate circuitry toprovide a single-chamber stimulation device capable of treating onechamber with cardioversion, defibrillation and pacing stimulation.

To provide atrial chamber pacing stimulation and sensing, thestimulation device 10 is shown in electrical communication with apatient's heart 12 by way of an implantable atrial lead 20 having anatrial tip electrode 22 and an atrial ring electrode 24 which typicallyis implanted in the patient's atrial appendage.

The stimulation device 10 is also shown in electrical communication withthe patient's heart 12 by way of an implantable ventricular lead 30having, in this embodiment, a ventricular tip electrode 32, aventricular ring electrode 34, a right ventricular (RV) coil electrode36, and an superior vena cava (SVC) coil electrode 38. Typically, theventricular lead 30 is transvenously inserted into the heart 12 so as toplace the RV coil electrode 36 in the right ventricular apex, and theSVC coil electrode 38 in the superior vena cava. Accordingly, theventricular lead 30 is capable of receiving cardiac signals, anddelivering stimulation in the form of pacing and shock therapy to theright ventricle.

While only two leads are shown in FIG. 3, it is to be understood thatadditional stimulation leads (with one or more pacing, sensing and/orshocking electrodes) may be used in order to efficiently and effectivelyprovide pacing stimulation to the left side of the heart or atrialcardioversion and/or defibrillation. For example, a lead designed forplacement in the coronary sinus region could be implanted to deliverleft atrial pacing, atrial shocking therapy, and/or for left ventricularpacing stimulation. For a complete description of a coronary sinus lead,see U.S. patent application Ser. No. 09/457,277, “A Self-Anchoring,Steerable Coronary Sinus Lead” (Pianca et al.), and U.S. Pat. No.5,466,254, “Coronary Sinus Lead with Atrial Sensing Capability”(Helland), which patents are hereby incorporated herein by reference.

The housing 40 (shown schematically) for the stimulation device 10includes a connector (not shown) having an atrial pin terminal 42 and anatrial ring terminal 44, which are adapted for connection to the atrialtip electrode 22 and the atrial ring electrode 24, respectively. Thehousing 40 further includes a ventricular pin terminal 52, a ventricularring terminal 54, a ventricular shocking terminal 56, and an SVCshocking terminal 58, which are adapted for connection to theventricular tip electrode 32, the ventricular ring electrode 34, the RVcoil electrode 36, and the SVC coil electrode 38, respectively. Thehousing 40 (often referred to as the “can”, “case” or “case electrode”)may be programmably selected to act as the return electrode, or anode,alone or in combination with one of the coil electrodes, 36 and 38. Forconvenience, the names of the electrodes are shown next to theterminals.

At the core of the stimulation device 10 is a programmablemicrocontroller 60 which controls the various modes of stimulationtherapy. As is well known in the art, the microcontroller 60 includes amicroprocessor, or equivalent control circuitry, designed specificallyfor controlling the delivery of stimulation therapy and may furtherinclude RAM or ROM memory, logic and timing circuitry, state machinecircuitry, and I/O circuitry. Typically, the microcontroller 60 includesthe ability to process or monitor input signals (data) as controlled bya program code stored in a designated block of memory. The details ofthe design and operation of the microcontroller 60 are not critical tothe present invention. Rather, any suitable microcontroller 60 may beused that carries out the functions described herein. The use ofmicroprocessor-based control circuits for performing timing and dataanalysis functions is well known in the art. As shown in FIG. 3, anatrial pulse generator 70 and a ventricular pulse generator 72 generatepacing stimulation pulses for delivery by the atrial lead 20 and theventricular lead 30, respectively, via a switch bank 74. The pulsegenerators, 70 and 72, are controlled by the microcontroller 60 viaappropriate control signals, 76 and 78, respectively, to trigger orinhibit the stimulation pulses. The microcontroller 60 further includestiming circuitry that controls the operation of the stimulation devicetiming of such stimulation pulses, that is known in the art. Thecontroller also includes a T-wave blanking system described in greaterdetail below.

The switch bank 74 includes a plurality of switches for switchablyconnecting the desired electrodes to the appropriate I/O circuits,thereby providing complete electrode programmability. Accordingly, theswitch bank 74, in response to a control signal 80 from themicrocontroller 60, determines the polarity of the stimulation pulses(e.g., unipolar or bipolar) by selectively closing the appropriatecombination of switches (not shown) as is known in the art. An atrialsense amplifier 82 and a ventricular sense amplifier 84 are also coupledto the atrial and ventricular leads 20 and 30, respectively, through theswitch bank 74 for detecting the presence of cardiac activity. Theswitch bank 74 determines the “sensing polarity” of the cardiac signalby selectively closing the appropriate switches, as is also known in theart. In this way, the clinician may program the sensing polarityindependent of the stimulation polarity.

Each sense amplifier, 82 and 84, preferably employs a low power,precision amplifier with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, known inthe art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the device 10 to deal effectively withthe difficult problem of sensing the low frequency, low amplitude signalcharacteristics of ventricular fibrillation. The outputs of the atrialand ventricular sense amplifiers, 82 and 84, are connected to themicrocontroller 60 which, in turn, inhibit the atrial and ventricularpulse generators, 70 and 72, respectively, in a demand fashion whenevercardiac activity is sensed in the respective chambers.

For arrhythmia detection, the invention utilizes the atrial andventricular sense amplifiers, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical depolarization,and “detection” is the processing of these sensed depolarization signalsand noting the presence of an arrhythmia. The timing intervals betweensensed events (e.g., P-P, R-R and R-T intervals) are then classified bythe microcontroller 60 by comparing them to a predefined rate zone limit(i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillationrate zones) and various other characteristics (e.g., sudden onset,stability, physiologic sensors, and morphology, etc.) in order todetermine the type of remedial therapy that is needed (e.g., bradycardiapacing, antitachycardia pacing, cardioversion shocks or defibrillationshocks, also known as “tiered therapy”).

Cardiac signals are also applied to the inputs of an analog to digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to acquire cardiac signals, convert the raw analog data intoa digital signal, and store the digital signals for later processingand/or telemetric transmission to an external device 102. The dataacquisition system 90 is coupled to the atrial and ventricular leads, 20and 30, through the switch bank 74 to sample cardiac signals across anypair of desired electrodes.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the stimulation device 10 to suit theneeds of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape and vector of each shocking pulse to bedelivered to the patient's heart 28 within each respective tier oftherapy.

Advantageously, the operating parameters of the implantable device 10may be non-invasively programmed into the memory 94 through a telemetrycircuit 100 in telemetric communication with an external device 102,such as a programmer, transtelephonic transceiver, or a diagnosticsystem analyzer. The telemetry circuit 100 is activated by themicrocontroller by a control signal 106. The telemetry circuit 100advantageously allows intracardiac electrograms and status informationrelating to the operation of the device 10 (as contained in themicrocontroller 60 or memory 94) to be sent to the external device 102through the established communication link 104.

In the preferred embodiment, the stimulation device 10 further includesa physiologic sensor 110. Such sensors are commonly called“rate-responsive” sensors. The physiological sensor 110 is used todetect the exercise state of the patient, to which the microcontroller60 responds by adjusting the rate and AV Delay at which the atrial andventricular pulse generators, 70 and 72, generate stimulation pulses.The type of sensor used is not critical to the invention and is shownonly for completeness. The stimulation device additionally includes abattery 114 which provides operating power to all of the circuits shownin FIG. 3. For the stimulation device 10, which employs shockingtherapy, the battery must be capable of operating at low current drainsfor long periods of time (preferably less than 10_A), and then becapable of providing high-current pulses (for capacitor charging) whenthe patient requires a shock pulse (preferably, in excess of 2 A, atvoltages above 2 V, for periods of 10 seconds or more). The battery 114must also have a predictable discharge characteristic so that electivereplacement time can be detected. Accordingly, the invention employslithium/silver vanadium oxide batteries, as is true for most (if notall) such devices to date. As further shown in FIG. 3, the inventionpreferably includes an impedance measuring circuit 120 which is enabledby the microcontroller 60 by a control signal 122. The impedancemeasuring circuit 120 is not critical to the invention and is shown foronly completeness.

Depending upon the implementation, the device may function as animplantable cardioverter/defibrillator (ICD) device. That is, it detectsthe occurrence of an arrhythmia, and automatically apply an appropriateelectrical shock therapy to the heart aimed at terminating the detectedarrhythmia. To this end, the microcontroller 60 further controls ashocking circuit 130 by way of a control signal 132. The shockingcircuit 130 generates shocking pulses of low (up to 0.5 Joules),moderate (0.5-10 Joules), or high energy (11 to 40 Joules), ascontrolled by the microcontroller 60. Such shocking pulses are appliedto the patient's heart through at least two shocking electrodes, and asshown in this embodiment, using the RV and SVC coil electrodes, 36 and38, respectively. In alternative embodiments, the housing 40 may act asan active electrode in combination with the RV electrode 36 alone, or aspart of a split electrical vector using the SVC coil electrode 38 (i.e.,using the RV electrode as common).

Cardioversion shocks are generally considered to be of low to moderateenergy level (so as to minimize pain felt by the patient), and/orsynchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5-40Joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 60 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

FIG. 4 is a flow chart illustrating the operation of the T-wave blankingmethod of the invention. In the flow chart, the various steps of themethod are summarized in individual “blocks”. Such blocks describespecific actions or decisions that are made or carried out as the methodproceeds. Where a microcontroller (or equivalent) is employed, the flowcharts presented herein provide the basis for a “control program” thatmay be used by such a microcontroller (or equivalent) to effectuate thedesired control of the stimulation device. Those skilled in the art mayreadily write such a control program based on the flow charts and otherdescriptions presented herein.

Within FIG. 4, at step 200, the implantable cardiac stimulation deviceinputs atrial and ventricular channel cardiac signals from a pair ofunipolar leads, one positioned within the atria, the other within theventricles. In the following, for the purposes of describing theconcepts of the T-wave blanking method of the invention, only a singleatrial channel and a single ventricular channel will be described. Theatrial channel is a channel from which the atrial rate is detected. Insome cases this may be also employed as an automatic mode switch channelor perhaps a bradycardia detection channel. In practical systems,numerous atrial and ventricular channels may be employed, each used fordifferent purposes. In any case, the atrial channel cardiac signalrepresents a voltage differential between the tip of the atrial lead andthe tip of the ventricular lead. The ventricular channel cardiac signalrepresents a voltage differential between the tip of the ventricularlead and the body of the implantable medical device. The device, at step202, filters the atrial and ventricular channels using combipolarfiltering techniques to eliminate noise and the like and to enhance thetrue atrial components of the atrial channel. Combipolar filteringtechniques are well known in the art and will not be described in detailherein. Further information may be found in the above-referenced patentto Hognelid.

To eliminate the R-waves and T-waves from the atrial channel, theimplantable device, at step 210 of FIG. 4, identifies R-waves on thecorresponding ventricular channel (not shown) and blanks any eventsdetected simultaneously on the atrial channel. Identification of R-wavesmay be performed using any conventional R-wave detection technique. Inthe event the medical device is currently pacing the ventricles, thenthe device merely ignores any events detected on the atrial channelsimultaneous with administration of pacing pulses to the heart. In thismanner, R-waves or ventricular paced events are blanked from the atrialchannel so as not to interfere with the correct determination of theatrial rate. Also, by identifying R-waves on the ventricular channel,the device can then begin to determine the expected location of theT-wave relative to the R-wave for blanking the atrial channel during theT-wave. To this end, the device, at step 212, inputs from stored memorythe expected R-T interval, i.e., the time interval between the R-waveand the expected T-wave. Preferably, the R-T interval represents theexpected amount of time between the end of the R-wave and the beginningof the T-wave. However, in other implementations, the R-T interval mayinstead represent the expected interval of time between the mid-point ofthe R-wave and the mid-point of the T-wave, or between any other pair ofidentifiable points within the R- and P-waves. At step 214, the devicethen inputs the expected duration of the T-waves from recorded memory.

The device then blanks a portion of the atrial channel, at step 216,using a T-wave blanking interval set using the expected R-T interval andthe expected T-wave duration. More specifically, the device utilizes atimer to track the expected R-T interval beginning at the end of theR-wave. Once the timer has elapsed, the device then ignores electricalsignals detected on the atrial channel, at least for the purposes ofatrial rate detection. When the blanking interval begins, the deviceactivates a second timer which times the expected duration of the T-waveas retrieved from recorded memory. Once the second timer has elapsed,the device then resumes detecting and processing signals on the atrialchannel at least for the purposes of atrial rate detection. The T-waveblanking interval is illustrated in FIG. 5 as interval 217.

At step 218, the device then analyzes the non-blanked portions of theatrial channel to determine the atrial rate. The atrial rate isdetermined, pursuant to conventional techniques, by tracking P-wavesdetected within non-blanked portions of the atrial channel. P-waves maybe detected, for example, by identifying all electrical events having amagnitude exceeding a predetermined threshold representative of themagnitude of P-waves. Since the R-waves are blanked at step 210 and theT-waves are blanked at step 216, any events exceeding the predeterminedP-wave threshold on the atrial channel most likely represent trueP-waves originating in the atria. The device counts the number ofP-waves during a predetermined period of time and determines therefromthe atrial rate.

At step 220, the device then administers or adjusts stimulation therapybased, in part, on the detected atrial rate. Within step 220, anyconventional pacemaker or ICD function affected by the detection ofdiscrete P-waves, the detection of the atrial rate, or the detection ofany other events on the atrial channel may be performed. By way ofexample, the device may administer anti-bradycardia pacing,anti-tachycardia pacing or other forms of pacing therapy or mayadminister cardioversion pulses to terminate fibrillation in the atriaor ventricles.

Within a state-of-the-art pacemaker or ICD, numerous functions performedby the device may be affected by the determination of the atrial rate orby other characteristics identified from the atrial channel of thecardiac signal. In some cases, the administration of therapy orperformance of other functions are triggered by detection of suchevents. In other cases, the administration of therapy or the performanceof other functions is inhibited by the detection of such events. Noattempt is made herein to identify the many functions which may beaffected by events detected on the atrial channel.

By eliminating only R-waves and T-waves from the atrial channel, thedetection of true atrial events and the true atrial rate is therebyimproved, which benefits the many functions affected by detection ofatrial events. This is in contrast with many conventional atrial channelblanking techniques, illustrated in FIGS. 1 and 2, wherein either allevents detected during a PVARP interval are ignored for the purposes ofatrial rate detection or wherein all events, including T-waves detectedduring the PVARP, are used for the purposes of atrial rate detection. Byeliminating all events during the PVARP interval, a significantunderestimation of the true atrial rate may occur. By using all eventsdetected during the PVARP an overestimation of the true atrial rate mayoccur.

Thus, what has been described with reference to FIG. 4, is a methodwhich utilizes a predetermined expected R-T interval and a predeterminedexpected T-wave duration to localize the expected position of T-waves onthe atrial channel for blanking thereof. With reference to FIG. 6, amethod for determining and updating the expected R-T interval and theexpected T-wave duration is illustrated. Briefly, the method of FIG. 6is periodically performed by the device to determine the average R-Tinterval and average T-wave duration for the patient in which the deviceis implanted. The device stores these averages in memory for use duringprocessing of the method of FIG. 4. The method of FIG. 6 may beperformed, for example, once per hour, once per day, or at any otherprogrammed frequency, as programmed by the physician. Alternatively, themethod of FIG. 6 may be performed on demand, perhaps if the devicebegins to detect anomalous behavior indicative of erroneous detection ofT-waves as being P-waves or it may be performed in response to the onsetof exercise or a change in posture or based or a significant change inheart rate. The method of FIG. 6 operate continuously updatinginformation as often as every cardiac cycle and thus providing a nearcontinuous estimate of the information acquired using the method of FIG.6.

In any case, at step 300, the device inputs atrial and ventricularchannel cardiac signals from unipolar leads and, at step 302, processesthe channels using combipolar filtering techniques to yield atrial andventricular channels having discrete and identifiable P-waves, R-waves,and T-waves. At step 304, the device identifies an R-wave on theventricular channel and determines its amplitude and othercharacteristics. Depending upon the specific programming of the system,the device may, for example, identify the maximum voltage, the minimumvoltage and the duration of the R-wave, along with the maximum andminimum slopes of the R-wave (i.e., the maximum and minimum firstderivatives of the instantaneous amplitude of the ventricular channelcardiac signal during the R-wave). At step 306, the device identifiesthe immediately succeeding T-wave on the ventricular channel anddetermines its amplitude and other characteristics.

Identification of a T-wave at step 306 may be determined, in accordancewith the invention, by identifying a relatively flat period of theventricular cardiac signal subsequent to the end of the R-wave, thendetecting an increase in the amplitude of the cardiac signal above apredetermined T-wave amplitude threshold. Alternative techniques may beemployed to compare portions of the ventricular channel cardiac signalwith predetermined templates representative of T-waves until a suitablematch is achieved. Note that the analysis is performed with respect tothe ventricular channel in which far-field atrial signals are likely tobe extremely low in amplitude. Hence, there is little or no risk that aP-wave detected on the ventricular channel will be misidentified as aT-wave. Depending upon the amount of noise in the system, however, thereis some risk that a noise artifact or some other anomalous electricalsignal, perhaps originating elsewhere in the body, may be initiallymisidentified as being a T-wave. Accordingly, steps, to be describedbelow, are provided to detect potentially erroneous T-waves so that theymay be eliminated from further processing.

At step 308, the device then determines the interval of time between theR-wave and the T-wave and further determines the duration of the T-wave.As noted above, the R-T interval is preferably set to equal the durationof time between the end of the R-wave and the beginning of the T-wavebut other specific intervals may alternatively be employed. Detection ofthe duration of the T-wave is performed, for example, by using apredetermined maximum noise threshold. Once the T-wave has beenidentified (perhaps based upon template matching or the like), thedevice then examines samples of the instantaneous voltage of the cardiacsignal immediately before and after the maximum amplitude point of theT-wave to identify all samples exceeding the maximum noise threshold.The duration of the T-wave is then defined to be the time period betweenthe point in time at which the ventricular cardiac signal rises abovethe maximum noise threshold to the point in time at which theventricular signal again drops below the maximum noise threshold. Othertechniques for defining and detecting the duration of the T-wave mayalternatively be employed.

At step 310, the device then determines whether its internal memoryalready includes recorded statistics representative of the average R-Tinterval and the average T-wave duration. If so, the device a inputs therecorded statistics representative of previously processed R-wave/T-wavepairs, including the mean and standard deviations of the R-T intervaland the T-wave duration. At step 312, the device then determines theextent to which the detected characteristics of the latest R-wave/T-wavepair deviate from the recorded statistics. If no statistics wereretrieved at step 310, step 312 is simply skipped. If recordedstatistics had been retrieved at step 310, and the characteristics ofthe latest R-wave/T-wave pair deviate significantly from the recordedstatistics as determined at step 313, then, at step 314, the deviceignores the latest R-wave/T-wave pair as being an anomalous pair.Otherwise, the device updates the recorded statistics, at step 316,using the characteristics of the latest R-wave/T-wave pair. For example,if at step 312, it is found that the R-T interval for the latestR-wave/T-wave pair is substantially greater than or less than theaverage R-T interval, then the device concludes that the latest pair ofevents represent an anomalous pair of events and the events arediscarded. Likewise, if the duration of the latest T-wave issubstantially greater than or less than the average duration of anaverage T-wave, then again the latest pair of events are discarded.Within step 312, other characteristics besides R-T interval and T-waveduration may be employed for the purposes of determining whether toupdate the prerecorded statistics. In general, any of thecharacteristics identified for the R-waves and T-waves may be employedfor making this determination. For example, a significant variationbetween the average maximum slope of an R-wave and the maximum slope ofthe newly detected R-wave may be employed to eliminate the R-wave fromfurther processing.

As can be appreciated, a wide range of characteristics or combinationsof characteristics may be compared with the predetermined statistics forthe purposes of discarding anomalous events. In making the determinationof step 313, the device preferably takes into account the currentventricular rate of the patient. In general, the R-T interval decreasesslightly with increasing ventricular rate. Accordingly, the R-T intervalof the newly detected pair of ventricular events is preferably adjustedto compensate for any change in interval caused solely by ventricularrate prior to comparison with the prerecorded average R-T interval. Inthe alternative, different average R-T intervals may be recorded fordifferent ranges of ventricular rates to permit the current R-T intervalto be compared directly with the average R-T interval for thecorresponding ventricular rate. As can be appreciated, numeroustechniques may be employed for compensating for variations in thecharacteristics of the ventricular events as a function of ventricularrate, or other characteristics of the state of the ventricles.

If, during steps 310 and 312, the device determines that there are noprerecorded statistics for ventricular events, then the device begins togenerate the statistics based upon the newly detected ventricularevents. Once some minimum number of ventricular events have beenprocessed sufficient to permit reliable statistics to be derived, suchas six to twenty events, the device then permits the recorded statisticsto be used for comparison against newly detected events at step 312. Inthis manner, the decision of step 313 is performed only if there is asufficient amount of data recorded to ensure reliable operation. In thealternative, the device may be preprogrammed with statisticsrepresentative of the R-T interval, T-wave duration and othercharacteristics for typical patients. In this manner, the device of anyparticular patient need not generate statistics entirely based upondetected events but may begin by utilizing the preprogrammed statistics.

In any case, with continuous or periodic processing of the steps of FIG.6, prerecorded statistics representative of the average R-T interval,T-wave duration and the like, are adjusted to take into account changesin these characteristics within the patient. Thus, if the patient takesany medications which may affect the R-T interval, the system willdetect the change in the average R-T interval, and will update itsrecorded statistics accordingly, such that, during processing of themethod of FIG. 4, the correct current expected R-T interval for thepatient is employed. Likewise, the average T-wave duration is adjustedperiodically or continuously within the device to permit the correctinterval to be employed during the processing of the steps of FIG. 4. Ingeneral, the system thereby provides a feedback mechanism for ensuringthat the expected R-T interval and expected T-wave duration arecontinuously readjusted to ensure correct blanking of T-waves from theatrial channel. Without this feedback mechanism, there is a risk thatthe blanking window employed within FIG. 4 would eventually deviate fromthe actual location of T-waves such that T-waves would begin to bedetected outside the blanking interval and erroneously misidentified asP-waves. By providing the aforementioned feedback mechanism, there islittle or no risk of this occurring and it is fairly certain that theT-wave blanking interval will almost always correspond with the actuallocation of T-waves on the atrial channel to permit reliable blankingthereof. Hence, the methods described herein provide an adaptive T-wavefilter which adapts to the changes in the patient. Other adaptivefiltering techniques may be employed to ensure that the R-T interval,the T-wave duration or any other characteristics used to localize theT-wave on the atrial channel.

The methods described in FIGS. 4-6 may be implemented, for example, byprogramming the controller of FIG. 3 to operate as a state machine inaccordance with the state machine diagram of FIG. 7. The state machinediagram of FIG. 7 generally corresponds with the methods alreadydescribed and hence the state machine will only be summarized.Initially, the controller transitions from starting state 400 to state402 wherein the controller performs operations directed to detecting anR-wave. If the controller remains in state 402 too long, e.g., the timein the R-WAVE WAIT state exceeds a threshold R-WAVE WAIT MAX, then ittransitions through state 404 to reset state 406 and ultimately back tostate 402. In this manner, R-waves detected after an inordinately longperiod of time are ignored for the purposes of T-wave blanking. While inthe state 402, the device operates to detect the onset of an R-wave bychecking for signal characteristics indicative of the onset of anR-wave, e.g., the absolute value of the R-wave, V(i), exceeds a limitR_THRESHOLD. If an R-wave is detected sufficiently promptly within state402, the controller transitions to state 408 where it performs functionsdirected to detecting the various characteristics of the R-waveincluding, for example, its maximum and minimum voltages, duration,maximum and minimum slopes, and the like. If the duration of the R-wavedetected during state 408 exceeds a predetermined threshold (e.g.,R-WAVE DURATION>R_WAVE DURATION MAX), indicating that the R-wave isprobably not a true R-wave but some other event, the controllertransitions through state 410 to the reset state and ultimately back tostate 402 for detection of another R-wave. Assuming that the duration ofthe R-wave detected during state 408 is not inordinately long, then thecontroller, upon completion of the R-wave, transitions to state 412where the controller waits for detection of a T-wave. One approach todetecting completion of an R-wave during state 408 uses a non-linearfilter operating on a digitally sampled R-wave signal. The R-wave issampled at about 250 samples per second. Absolute values of differencesbetween samples separated by 0.008 and 0.025 seconds are averaged over0.01 to 0.04 seconds. These averaged differences, R_DIFF_SUM, seem tovery effective at detecting the completion of an R-wave, e.g.,R_DIFF_SUM<R_DIFF_THRESHOLD.

During state 412, the controller performs functions directed todetecting the next T-wave including, for example, monitoring theamplitude and slope of the ventricular cardiac signal until theamplitude and slope varies in accordance with the expectedcharacteristics of the beginning of a T-wave (V(i)>T-WAVE THRESHOLD,where V(i) is, e.g., the amplitude or slope of the ventricular cardiacsignal). If the controller remains in state 412 for too long (T-WAVEWAIT>MAX_T-WAVE WAIT), indicating a possibly anomalous event, thecontroller transitions through state 414 to reset state 406 andultimately back to state 402 to detect another R-wave. If a T-wave isdetected sufficiently promptly within state 412, the controllertransitions to state 416 wherein the controller performs functionsdirected to processing the T-wave including, for example, detecting theamplitude and duration of the T-wave, as well as its maximum and minimumslopes and the like. If the duration of the T-wave detected within state416 is too long, indicating that the event is a true T-wave, thecontroller transitions through state 418 to reset state 406 andultimately back to state 402 for detection of yet another R-wave. If,however, the duration of the T-wave does not exceed the maximum expectedduration and the completion of the T-wave is detected, the controllertransitions to state 420. One approach to detecting completion of aT-wave during state 416 uses a non-linear filter operating on the adigitally sampled T-wave signal. The T-wave is sampled at about 250samples per second. Absolute values of differences between samplesseparated by 0.008 and 0.032 seconds are averaged over 0.01 to 0.064seconds. These averaged differences, T_DIFF_SUM, seem to very effectiveat detecting the completion of an R-wave, e.g.,T_DIFF_SUM<T_DIFF_THRESHOLD.

While in state 420, the controller performs functions directed togenerating statistics or updating statistics for R-wave/T-wave pairs.Once statistics have been generated or updated, the controllertransitions to reset state 406, and ultimately to state 402 fordetecting another R-wave.

Thus, FIG. 7 illustrates the various states of the controller for use indetecting R-wave/T-wave pairs and for updating the statisticalrepresentation thereof. This represents one example of the programmingof the controller of the medical device. In the alternative, thecontroller may be programmed in accordance with other specific statemachines or in accordance with other techniques to perform the generalfunctions outlined above.

As can be appreciated, a wide variety of techniques are consistent withthe general principles of the invention. The embodiments describedherein are merely illustrative of aspects of the invention and shouldnot be construed as limiting the scope of the invention which is to beinterpreted in accordance with the claims that follow.

What is claimed is:
 1. A method performed by an implantable cardiacstimulation device for blanking a portion of a cardiac signal, themethod comprising: sensing a cardiac signal; identifying an expectedlocation and duration of a T-wave within the cardiac signal; andblanking a portion of an atrial channel of the cardiac signal to ignoresignals occurring within a period of time corresponding to the expectedlocation and duration of the T-wave.
 2. The method of claim 1 whereinthe implantable cardiac stimulation device comprises unipolar leadsadapted to be coupled to heart tissue and wherein sensing a cardiacsignal comprises: sensing atrial and ventricular channel cardiac signalsusing the unipolar leads; and filtering the atrial and ventricularchannel cardiac signals using combipolar filtering.
 3. The method ofclaim 1 further comprising: analyzing non-blanked portions of the atrialchannel of the cardiac signal to determine the behavior of the heart. 4.The method of claim 3 wherein analyzing the non-blanked portions of theatrial channel comprises detecting P-waves therein.
 5. The method ofclaim 3 wherein analyzing the non-blanked portions of the atrial channelof the cardiac comprises detecting the atrial rate.
 6. The method ofclaim 3 wherein analyzing the non-blanked portions of the atrial channelcomprises detecting a dysrhythmia, if any, in the heart.
 7. The methodof claim 6 wherein the dysrhythmia is one of atrial flutter and atrialfibrillation.
 8. The method of claim 3 further comprising: controllingstimulation operations of the device based on the analysis of thenon-blanked portions of the atrial channel of the cardiac signal.
 9. Themethod of claim 8 wherein controlling stimulation operations comprisesdetermining whether to trigger an auto-mode switching operation withinthe implantable cardiac pacing device.
 10. The method of claim 1 whereinidentifying the expected location and duration of a T-wave within thecardiac signal comprises: identifying an R-wave in the cardiac signal;tracking a first predetermined interval of time subsequent to theR-wave, the first predetermined time interval being representative of anexpected interval between the end of an R-wave and the beginning of asuccessive T-wave; and tracking a second predetermined interval of timesubsequent to the beginning of the T-wave, the second predetermined timeinterval being representative of an expected duration of a T-wave. 11.The method of claim 10 further comprising: determining the firstinterval of time by measuring and averaging the intervals between aplurality of R-wave/T-wave pairs; and determining the secondpredetermined interval of time by measuring and averaging the durationsof a plurality of T-waves.
 12. A system for blanking a portion of acardiac signal using an implantable cardiac stimulation device, thesystem comprising: means for sensing a cardiac signal; means foridentifying an expected location and duration of a T-wave within thecardiac signal; and means for blanking portions of an atrial channel ofthe cardiac signal to ignore signals occurring within a period of timecorresponding to the expected location and duration of the T-wave.
 13. Asystem for blanking a portion of a cardiac signal using an implantablecardiac stimulation device, the system comprising: a sensing system forsensing a cardiac signal; and a controller for identifying an expectedlocation and duration of a T-wave within the cardiac signal and forblanking portions of an atrial channel of the cardiac signal to ignoresignals occurring within a period of time corresponding to the expectedlocation and duration of T-waves.